Amplifying system



March 1.0, 1931. H. WHITTLE ETAL AMPLIFYING SYSTEM Filed Oct. 26. 1927 B5 FREQUENCY Kl]. OCYCLEJ Homo/5 WH/T m5 WVENTORSAR THUR J. GHR/s TOPHER A 7' TORNE Y Patented Mar. 10, 1931 UNITED STATES PATENT OFFECE HORACE WHITTLE, OF MAPLEWOOD, NEW JERSEY, AND ARTHUR J'. CHRISTOPHER,

OF YONKERS, NEW YORK, ASSIGNORS TO BELL TELEPHONE LABORATORIES, IN- CORPORATEI), OF NEW YORK, N. Y., A CORPORATION OF NEW YORK AMPLIFYING SYSTEM Application filed October 26, 1927. Serial No. 228,727.

This invention relates to amplifying systems and particularly to networks used therein to couple two space discharge devices in tandem.

An object of the invention is to secure a uniform voltage transformation of waves within a prescribed band of frequencies with a sharp attenuation of frequencies immedi ately outside of the prescribed band.

Another object is to increase the overall amplification by a space discharge amplifier of Waves within a prescribed band of frequencies to the maximum degree consistent with uniform amplification throughout the band and a high degree of selectivity.

The above objects are accomplished in accordance with the invention by utilizing for coupling two space discharge devices in a multi-stage amplifier, a network comprising two tuned transformers coupled in tandem through a series condenser, so that the complete network comprises three coupled tuned circuits, the first and last being shunt antiresonant circuits and the intermediate being a closed series resonant circuit. These circuits are all tuned alike to the same frequency, which is preferably near the center of the selected band. By properly selecting the relative values of the elements of the coupled circuits and proportioning the coupling coefficients of the network in accordance with the present invention, it is found that all waves within a predetermined band of frequencies can be transformed and transmitted by the network with maximum an1plification and in a substantially uniform degree, while waves of frequencies outside the limits of the band will be strongly attenuated.

The coupling circuit of the invention besides combining the features of high amplification and great selectivity is advantageous from the standpoint of its simplicity, small size and low cost of manufacture, as it comprises a small number of elements which may be economically made commercially, and which are arranged to take up a minimum of space.

The nature of the invention and the manner of its application is explained more fully cuits 10 and 11.

in the following description in connection with the accompanying drawings in which:

Fig. 1 shows conventionally in schematic form a portion of the circuit of a space discharge amplifier embodying the invention;

Fig. 2 is a detailed view of one embodiment of the coupling circuit used in the circuit of Fig. 1, showing the detail features and arrangement of the parts making up the coupling circuit; and

Fig. 3 shows an actual voltage amplification characteristic obtained with a circuit such as shown in Fig. 1 embodying the in- Vention.

The portion of the amplifier circuit shown in 1 comprises a space discharge amplifier tube 1 and a space discharge tube 2, acting as a grid bias detector, both of the wellknown three-electrode type, connected in tandem between input terminals 3 and an output transformer 4. The output circuit of space discharge amplifier tube 1 is connected to the input circuit of space discharge detector tube 2 by a coupling network 9.

The network 9 comprises a shunt anti-resonant circuit 10 connected between points 5 and 6 in the output circuit of amplifier tube 1, another shunt anti-resonant circuit 11 connected between points 7 and 8 in the input circuit of detector tube 2, and a closed series resonant circuit 12 magnetically and electrically coupling the anti-resonant cir- The anti-resonant circuit 1O comprises the primary winding 13 of transformer 14 and a condenser 17, in shunt therewith. The closed series resonant circuit 12 comprises the secondary winding 15 of transformer 14, the series condenser 17 and the primary winding 18 of transformer 19. The anti-resonant circuit 11 comprises the secondary winding 20 of transformer 19 and the total capacity effectively in shunt therewith, as indicated by the dotted condenser 21, which comprises the effective input capacity of detector tube 2 plus the distributed capacity between the terminals of secondary winding 20 of transformer 19. The amplifier tube 1 and the detector tube 2 have the usual space current supply, grid polarizing and filament heating batteries.

The condenser 22 connected across the primary winding of output transformer 1 provided to serve as a low impedance to the carrier frequencies in the output circuit of detector tube 2.

The method of selecting the actual values of the various elements in coupling circuit 9 to obtain the desired high amplification with uniform transmission efficiency for a given band of frequencies, and sharp attenuation for the frequencies immediately outside the given band, will be explained here inafter.

In Fig. 2 is shown the detailed features and general arrangement of one embodiment of the coupling network 9 of Fig. 1, the elements therein corresponding-to the elements shown schematically in the circuit of Fig. 1 bearing corresponding de'signationcharacters.

Referring to Fig. 2, it will be noted that the windings 18 and 15 of transformer 1 1 are wound side by side in parallel grooves of a wooden spool 23. The priman Winding 13 and the secondary winding 15 of transformer 14. are approximately equal electrically, that is, they have approximately the same inductance and distributed capacities, and each is designed to obtain maximum ratio of reactance to resistance over the trans Pitt-6C1 band, such as by using stranded wir in place of a solid conductor of equivalent cross-section.

The windings 1S and 20 of transformer 19 are wound side by side in parallel grooves of a wooden spool 2 l. he primary winding 18 and the secondary 20 of transform r 19 are electrically unequal, the primary W 1nding 18 being substantially equal electrically to the secondary winding 15 of transformer 14. The secondary winding 20 of tra r former 19 is wound in two sections narrow grooves in spool 24. Since the object-of the coupling network 9 to increase the amplification or the selected band of frequencies as much as possible, it is desirable to reduce to a minimum the listributed capacity and dielectric losses of winding 20 operating into the input circuit of detector 2. This is done in this particular case by Winding secondary winding 20 in the two separated narrow grooves in spool The electromagnetic coupling between trai formers 1a and 19 is reduced to a minimum by arranging the spools 23 and 24, as shown in Fig. 2, so that the nagnetic fielcs of the transformer windings wound thereon are at right angles with each other. The condenser box 17 consists of two individual capacities 17 and 17 separated electrically from each other, the capacity 17;, being connected in series with secondary winding 15 of trans former 14 and the primary winding 18 of transformer 19, and the capacity 1? being connected in shunt with the primary winding 13 of transformer 14, as indicated in the schematic circuit of Fig. 1. The network 9 may be shielded from other apparatus in the circuit by placing it in a copper case (not shown).

The method of designing coupling circuit 9 to accomplish the objects of the invention as set forth above will be clear from the followin theoretrical considerations.

it w'll be noted that the coup-ling network 9 of Figs. 1 and2 is similar to, and for purposes of design may be'considered equivalent to two-half-sectio-n confluent band pass filters.

and the effective capacity C in shunt to the primary winding 13 of transformer 14 is where R equals the impedance looking into the filter for waves of the frequency f, and is assumed to be equal or preferably a little less than the impedance from which the filter operates; and F and F are respectively the upper and lower cut-off frequencies of the filter.

Actually, in such a filter structure the position of the upper and lower cut-0E frequencies will occur outside of the flat portion of its attenuation-frequency characteristic. In the particular case for which the circuit of the invention was designed, it was desired to transmit with substantially uniform transmission efliciency and maximum amplification, a frequency band ranging from 78 kilocycles to 88 kilocycles, the mid-frequency of the band being 83 lrilocycles. It was assumed therefore, that the upper cut-0E of the filter structure should be set at approximately 91 kilocycles and the lower cut-off at approximately 7 5 lrilocycles. The impedance looking into the filter at'mid-frequency was assumed to be 17,090 ohms, which is the plate circuit impedance of a particular type of vacuum tube having such characteristics as to be suitable for the amplifier tube 1 feeding into the coupling network 9. Substituting the values given in Equation (1) it is found that =5s5 mmf. a

was such as to make the total effective capacity approach the theoretical capacity as computed above. The inductance, L of the primary winding 13 of transformer 14: which will resonate with the 500 mmf. condenser in shunt therewith plus the distributed capacity of the winding at the frequency f of the band to be transmitted, may be computed from the formula 1 Tr j 01 (4) where C equals 0+ 0 or the capacity 500 mmf. plus the distributed capacity of the winding 13.

As stated above, the inductances of the secondary winding 15 of transformer 14: and the primary winding 18 of transformer 19, which in the following equations will be referred to respectively as L and L are equal. Their combined inductance should be such as to resonate with a capacity equal to at the frequency 7. Therefore, a condenser having the capacity of or 250 mmf. may be utilized for condenser 17 b connected in series with windings 15 and 17 in coupling network 9. The distributed capacity of windings 15 and 17 may be neglected in high time constant coils provided it is not too large.

The inductance, L,, of the secondary winding 20 of transformer 19 is adjusted so that it will resonate with the total capacity, C effectively in shunt therewith, which comprises the effective input capacity of detector tube 2 plus the distributed capacity between the turns of winding 20, at the frequency f of the selected band. This inductance may be computed from the following formula where is in kilocycles, and C equals the effective input capacity of tube 2 plus the effective distributed capacity of winding 20 in mmf.

The electromagnetic coupling between transformers 14 and 19 should be reduced to a minimum. One way of doing this is to mount these transformers so that their fields are at substantially right angles to each other.

The desirable coupling factor K for the transformers 14 and 19 may be determined experimentally or may be computed from the following formula Substituting the values for the upper and lower cut-off frequencies as assumed above when the value of G was determined, of 91 kilocycles for F and 7 5 kilocycles for F it is seen that The derivations of the above formulae used in designing the coupling network 9 are given in a previous Patent, No. 1,714,149, issued to the applicants on May 21, 1929. Combining Equations (2) and (4), the following equation is obtained:

Combining Equations (6) and (8), it is found that 2 2 2 g i fi 2F2F1 (9) The voltage step-up Z of network 9 is Then, since networks 10, 11 and 12 are all tuned to the same frequency It is desirable, and has been found to be commercially possible, to adjust the inductances of the windings in the coupling network 9 to i 1% and the capacities of the condensers in network 9 to i 2% in order to insure the proper location of the frequency band as well as uniform voltage amplification over the band. I

The coupling coefiicients for network 9 can be kept within the desired limits by placing limits on the dimensions of the separations between the windings of transformer 14 and of transformer 19.

An actual voltage amplification characteristic obtained with a circuit such as shown in Fig. 1 in which the values of the elements of the coupling network 9 and their relative positions were determined by the method of design given above, is shown in Fig. 3 of the drawing. Referring to F 8, it will be noted that the voltage amplification Pi /E between the input of tube 1 and the input of tube 2 of the circuit of Fig. 1, is approximately 32 times over a frequency band ranging from 7 8 to 88 kilocycles, that the variation over this band corresponds to less than 1 TU, and that the voltage amplification outside the operating range decreases rapidly and uniformly, the gain at points approximately 12,500 cycles from the mid-frequency of the band being approximately 20 transmission units (TU)' below that at mid-frequency,which corrix responds to a reduction in the voltage am p'lilication of approximately one-tenth at these'points. (A transmission unit (TU) is defined by specifying that the number of transmission units corresponding to'a power ratio of P /P is equal to 10 log P /P The coupling network 9 of the invention, as described above, is particularly adaptable for use in amplifier systems designed to amplify selectively waves of a band of frequencies.

F or example, in radio telephone receiving v i understood that it is not to be limited to the particular circuit shown as a whole or to the specific details described, but only by the scope of the appended claims.

What is claimed is: v V

1. A wave transmission system comprising in combination with a wave source and a wave receiver having unequal impedances, means for transmitting from said source to said iver waves of a predetermined band of uencies with a maximum voltage ampliication for the frequencies in said band con 1 with a desired quality of transmission, means comprising a coupling network consisting of a pair of anti-resonant circuits intermediate closed series resonant circuit coupled in tandem between said source and said receiver, the constants of said network being related to each other and to the bar; of *equencies to be amplified in accordance with the equation ZU F F K 2 1 where L is the inductance and C the capacity effectively in shunt therewith in the anti resonant circuit connected to said source, L and L, the inductances and C the capacity in said intermediate resonant circuit, L the inductance and C the capacity effectively in shunt therewith in the anti-resonant circuit connected to said re -eiver, Z is L limiting frequencies of said band.

2. A system in accordance with claim 1 and .in which the values of the constants of said coupling network are such that the network produces a high degree of attenuation in the frequencies received from said wave source lying outside the range of said band.

3. A system in accordance with claim 1, and in which the values of the constants of said network are such that said band of frequencies has a width of approximately ten kilocycles. I

4:. A system in accordance with claim 1, and in which the values of the constants of said network are such that said band of freuencies extends from a frequency of approximately 78 kilocycles to a free uoncy of 7 approximately 88 kilocycles.

5. In a wave transmission system for transmitting waves of a plurality of frequencies comprising an amplifying circuit consisting of two space discharge devices and a coupling network therebetween comprising an anti-resonant circuit connected to the output of one of said devices, a second anti-resonant circuit connected to the input of the other of said devices, and a closed series resonant circuit magnetically and electrically coupling the anti-resonant circuits,said antiresonant circuits and said resonant circuit being tuned to substantially the same fre quency, the coupling and impedance coefficients of said network being proportioned with respect to each other and a predeterminedc'band of frequencies to be amplified extending on either side of said same frequency so that saidnetwork transmits with substantially uniform transmission efficiency and with maximum amplification-said band of frequencies and produces a high degree of attenuation in the frequencies outside said band.

'6. A wave transmission system in accordance with claim 1, and in which the values of the constants of the said network and its associated circuits such that said network transmits with substantially uniform transmission eiiiciency a band of frequencies having a range of approximately ten kilocycles, and in which the attenuation produced by said network on the transmitted frequencies outside said ran e and approximately twelve kilocycles irom the mid-frequency of said band is at least twenty transmission units TU) below the attenuation produced by said network on said mid-frequency.

7. A wave transmission system comprising in combination with a source of waves having a wide ran e of freouencies and a wave reb V l 7 ceiver havinga substantially capacitive input primary winding and forming therewith a shunt anti-resonant circuit, a second transformer having a primary winding of inductance L and a secondary winding of inductance L connected to the input of said wave receiver and forming with the capacitive input impedance thereof a second shunt antiresonant circuit, a second capacitive impedance element of value C said second capacitive element being connected in series in a closed circuit with the secondary winding of the first mentioned transformer and the primary winding of said second transformer and forming therewith a series resonant circuit, the constants of said network being related to each other and to the frequencies of a predetermined band of frequencies to be transmitted by said network in accordance with the equation where 0 and C are the capacities respectively in shunt with said primary winding of the first mentioned transformer and the secondary winding of said second transformer, Z is R is the effective impedance of said source, and F 2 and F are the limiting frequencies in said predetermined band, said network transmits between said source and said receiver with maximum amplification for a given quality of transmission for said predeten mined band of frequencies, while producing rapidly increasing attenuation to the frequencies adjacent to said band.

8. A system in accordance with claim 5 and in which said anti-resonant circuits are arranged with respect to each other so as to reduce the electromagnetic coupling between said anti-resonant circuits.

In witness whereof, we hereunto subscribe our names this 24 day of October, A. D. 1927.

HORACE WHITTLE. ARTHUR J. CHRISTOPHER. 

